Interaction of Trypanosoma cruzi adenylate cyclase with liver regulatory factors

Cyclic-nucleotide signalling in protozoa

Compared with the impressive progress in understanding signal transduction pathways and mechanisms in mammalian systems, advances in protozoan signalling processes, including cyclic nucleotide metabolism, have been very slow. This is in large part connected to the fact that the components of these pathways are very different in the protozoan parasites, as confirmed by the recently completed genome. For instance, kinetoplastids have no equivalents to the mammalian Class I adenylyl cyclases (ACs) in their genomes nor any of the subunits of the associated G-proteins. The cyclases in kinetoplastid parasites contain a single transmembrane domain, a conserved intracellular catalytic domain and a highly variable extracellular domain - consistent with the expression of multiple receptor-activated cyclases - but no receptor ligands, agonists or antagonists have been identified. Apicomplexan AC and guanylyl cyclase (GC) are even more unusual, potentially being bifunctional, harbouring either a putative ion channel (AC) or a P-type ATPase-like domain (GC) alongside the catalytic region. Phosphodiesterases (PDEs) and cyclic-nucleotide-activated protein kinases are essentially conserved in protozoa, although mostly insensitive to inhibitors of the mammalian proteins. Some of the PDEs have now been validated as promising drug targets. In the following manuscript, we will summarize the existing literature on cAMP and cGMP in protozoa: cyclases, PDEs and cyclic-nucleotide-dependent kinases.

Signal transduction mechanisms in Trypanosoma cruzi

Trypanosoma cruzi, the etiological agent of Chagas disease, is an adequate model for studies on the evolution of signal transduction pathways. These pathways involve molecular entities such as membrane receptors, transduction G proteins, protein kinases and second messengers (Ca(2+), cyclic AMP, cyclic GMP, nitric oxide). In this article, Mirtha M. Flawiá, María T. Téllez-Iñón and Héctor N. Torres describe the studies performed on T. cruzi transduction pathways and their role in the control of metacyclogenesis and cell motility.

A novel calcium-stimulated adenylyl cyclase from Trypanosoma cruzi, which interacts with the structural flagellar protein paraflagellar rod

Trypanosoma cruzi adenylyl cyclases are encoded by a large polymorphic gene family. Although several genes have been identified in this parasite, little is known about the properties and regulation of these enzymes. Here we report the cloning and characterization of TczAC, a novel member of T. cruzi adenylyl cyclase family. The TczAC gene is expressed in all of the parasite life forms and encodes a 1,313-amino acid protein that can complement a Saccharomyces cerevisiae mutant deficient in adenylyl cyclase activity. The recombinant enzyme expressed in yeasts is constitutively active, has a low affinity for ATP (K(m) = 406 microm), and requires a divalent cation for catalysis. TczAC is inhibited by Zn(2+) and the P-site inhibitor 2'-deoxyadenosine 3'-monophosphate, suggesting some level of conservation in the catalytic mechanism with mammalian adenylyl cyclases. It shows a dose-dependent stimulation by Ca(2+) which can be reversed by high concentrations of phenothiazinic calmodulin inhibitors. However, bovine calmodulin fails to stimulate the enzyme. Using a yeast two-hybrid screen it was found that TczAC interacts through its catalytic domain with the paraflagellar rod protein, a component of the flagellar structure. Furthermore, we demonstrate that TczAC can dimerize through the same domain. These results provide novel evidence of the possible localization and regulation of this protein.

G-protein from Medicago sativa: functional association to photoreceptors

G-protein subunits were characterized from Medicago sativa (alfalfa) seedlings. Crude membranes and GTP-Sepharose-purified fractions were electrophoresed on SDS/polyacrylamide gels and analysed by Western blotting with 9193 (anti-alpha common) and AS/7 (anti-alpha t, anti-alpha i1 and anti-alpha i2) polyclonal antibodies. These procedures led to the identification of a specific polypeptide band of about 43 kDa. Another polypeptide reacting with the SW/1 (anti-beta) antibody, of about 37 kDa, was also detected. The 43 kDa polypeptide bound specifically [alpha-32P]GTP by a photoaffinity reaction and was ADP-ribosylated by activated cholera toxin, but not by pertussis toxin. Irradiation of etiolated Medicago sativa protoplast preparations at 660 nm for 1 min produced a maximal increase in the guanosine 5'-[gamma-thio]triphosphate (GTP[35S])-binding rate. After this period of irradiation, the binding rate tended to decrease. The effect of a red-light (660 nm) pulse on the binding rate was reversed when it was immediately followed by a period of far-red (> 730 nm) illumination. These results may suggest that activation of GTP[S]-binding rate was a consequence of conversion of phytochrome Pr into the Ptr form.

Pathways of the evolution of hormonal signal realization systems

The problem of the structural-functional organization, and of the origin and evolution of the chemosignal systems which realize the effect of hormones and hormone-like substances in the higher eukaryotes-lower eukaryotes-prokaryotes series, is reviewed on the basis of an analysis of published information and our own data. The notion that the systems of the conduction and transduction of chemical signals are related to universal and evolutionarily ancient structures is formulated. The roots of these systems take their origin in the single-celled eukaryotes, while individual of their functional units take their origin even in the prokaryotes. The hypothesis is advanced that the progressive evolution of chemosignal systems has traveled common pathways, and has consisted in the linking up and the functional combination of originally general and conservative units in the direction of the ever-increasing specialization of these systems and augmentation of their efficiency.

Adenylyl cyclase activity of the fission yeast Schizosaccharomyces pombe is not regulated by guanyl nucleotides

The adenylyl cyclase activity of the fission yeast Schizosaccharomyces pombe is localized to the plasma membrane of the cell. The enzyme utilizes Mn2+/ATP as substrate and free Mn2+ ions as an effector. Unlike the baker yeast Saccharomyces cerevisiae, S. pombe adenylyl cyclase does not utilize Mg2+/ATP as substrate and the activity is not stimulated by guanyl nucleotides. The optimal pH for the S. pombe adenylyl cyclase activity is 6.0. The activity dependence on ATP is cooperative with a Hill coefficient of 1.68 +/- 0.14.

Reconstitution of a light-stimulated adenylate cyclase from retina and Neurospora crassa preparations. Characterization of the heterologous systems using normal and degenerative retinas

Adenylate cyclase catalytic subunits from Neurospora crassa membranes may interact with regulatory factors from membranes of bovine retinal rod outer segments (pretreated with N-ethylmaleimide), reconstituting a heterologous system which, in the presence of light, is catalytically active in assay mixtures containing MgATP. Maximal activation was observed at 550 nm. Transducin-depleted retinal membranes were not capable of reconstituting the heterologous light-stimulated adenylate cyclase system. Addition of a transducin preparation to depleted membranes restored the reconstitution capacity of these membranes. A similar heterologous adenylate cyclase system was reconstituted with Neurospora and mouse retinal whole membranes (pretreated with N-ethylmaleimide). Membranes from mice suffering photoreceptor degeneration (rd homozygotes) did not reconstitute an heterologous adenylate cyclase system.

The evolutionary origin of eukaryotic transmembrane signal transduction

1. A comparison was made of transmembrane signal transduction mechanisms in different eukaryotes and prokaryotes. 2. Much attention was given to eukaryotic microbes and their signal transduction mechanisms, since these organisms are intermediate in complexity between animals, plants and bacteria. 3. Signal transduction mechanisms in eukaryotic microbes, however, do not appear to be intermediate between those in animals, plants and bacteria, but show features characteristic of the higher eukaryotes. 4. These similarities include the regulation of receptor function, adenylate cyclase activity, the presence of a phosphatidylinositol cycle and of GTP-binding regulatory proteins. 5. It is proposed that the signal transduction systems known to operate in present-day eukaryotes evolved in the earliest eukaryotic cells.

Trypanosoma cruzi: adrenergic modulation of cyclic AMP role in proliferation and differentiation of amastigotes in vitro

Trypanosoma cruzi amastigotes, obtained from the supernatant of J774G-8 macrophage cultures infected with Y strain trypomastigotes, proliferated and differentiated into epimastigotes in Warren medium at 28-29 C. The basal level of adenosine 3':5'-monophosphate (cAMP) in recently harvested amastigotes was 0.12 pmole/10(7) cells, which could be increased in a dose-dependent manner to 0.62 pmole/10(7) cells with 1 mM of the adrenergic ligand isoproterenol plus 0.5 mM isobutyl methylxanthine. Isoproterenol inhibited [3H]thymidine incorporation into amastigote DNA, as well as the proliferation of amastigotes and newly transformed epimastigotes. Because dibutyryl cAMP had the same effect as isoproterenol on the cells, the experimental results suggest a role for cAMP, modulated by adrenergic ligands, in the control of proliferation and differentiation of amastigotes.

Regulation of ciliary adenylate cyclase by Ca2+ in Paramecium

In the ciliated protozoan Paramecium, Ca2+ and cyclic nucleotides are believed to act as second messengers in the regulation of the ciliary beat. Ciliary adenylate cyclase was activated 20-30-fold (half-maximal at 0.8 microM) and inhibited by higher concentrations (10-20 microM) of free Ca2+ ion. Ca2+ activation was the result of an increase in Vmax., not a change in Km for ATP. The activation by Ca2+ was seen only with Mg2+ATP as substrate; with Mn2+ATP the basal adenylate cyclase activity was 10-20-fold above that with Mg2+ATP, and there was no further activation by Ca2+. The stimulation by Ca2+ of the enzyme in cilia and ciliary membranes was blocked by the calmodulin antagonists calmidazolium (half-inhibition at 5 microM), trifluoperazine (70 microM) and W-7 (50-100 microM). When ciliary membranes (which contained most of the ciliary adenylate cyclase) were prepared in the presence of Ca2+, their adenylate cyclase was insensitive to Ca2+ in the assay. However, the inclusion of EGTA in buffers used for fractionation of cilia resulted in full retention of Ca2+-sensitivity by the ciliary membrane adenylate cyclase. The membrane-active agent saponin specifically suppressed the Ca2+-dependent adenylate cyclase without inhibiting basal activity with Mg2+ATP or Mn2+ATP. The ciliary adenylate cyclase was shown to be distinct from the Ca2+-dependent guanylate cyclase; the two activities had different kinetic parameters and different responses to added calmodulin and calmodulin antagonists. Our results suggest that Ca2+ influx through the voltage-sensitive Ca2+ channels in the ciliary membrane may influence intraciliary cyclic AMP concentrations by regulating adenylate cyclase.

Evidence for the existence of an Ns-type regulatory protein in Trypanosoma cruzi membranes

The existence of a GTP-binding protein of the Ns type in Trypanosoma cruzi was explored. Epimastigote membranes were labelled by cholera toxin in the presence of [adenine-14C]NAD+. After SDS/polyacrylamide-gel electrophoresis of extracted membrane proteins, a single labelled polypeptide band of apparent Mr approx. 45,000 was detected. Epimastigote cells were treated with N-ethylmaleimide and electrofused to lymphoma S49 cells lacking the Ns protein. Evidence indicates that in such electrofusion-generated cell hybrids a heterologous adenylate cyclase system was reconstituted with the Ns protein provided by T. cruzi epimastigotes.